Patent GB-678716-A (GB)
Title: Improvements in television receivers
IPC: H04N5/46
Filed: 1949-04-26
Abstract: 678,716. Television receivers. RADIOINDUSTRIE SOC. ANON. April 26, 1949 [April 30, 1948; Oct. 9, 1948], Nos. 11065/49 and 11066/49. Class 40 (iii). In a television receiver arranged to be oper. able in response to one or the other of two broadcasts corresponding to different definitions, high and low, of the picture or object and transmitted with different band widths on separate and distinct carriers with additional separate and distinct carriers for the accompanying sound, and including high-frequency stages with a tuned circuit, a local oscillator adjusted by another tuned circuit, an intermediate amplifier of predetermined fixed frequency with a wide band-pass and adjustable scanning-voltage generators, a switching means effective to change from one to the other of the two broadcasts varies the tuning circuits of the local oscillators, the scanning circuits and the tuned circuits of the high-frequency stages. The latter switching operation introduces in the case of the high-definition broadcast, a highly damped tuning circuit to affect the middle of the intermediate band-pass, and, in the case of the low-definition broadcast, a lightly damped circuit having a resonance frequency close to the highest frequency of the corresponding low-definition transmission spectrum. As shown in Fig. 1, the receiver comprises a circuit 1 including L.F. circuits and local oscillators, intermediate frequency bandpass amplifiers 2, image detector 3, videofrequency stage 4 and cathode-ray receiver 5. The video-frequency is also fed to a separate unit 6 and a unit 7 for separating frame and line pulses which are fed to oscillators 8, 9 controlling the vertical and horizontal scanning. The sound-receiving circuit comprises the sound intermediate frequency amplifier 10, detector 11, L.F. amplifier 12 and loud-speaker 13. A switch member 14 controls over connection 101 the L.F. tuned circuits and local oscillators and at 102, 103 controls a capacitor to vary the frequency of the line oscillator and means to restore the scanning amplitude to its normal value. The connection 104 introduces appropriate circuits in the event of the sound being transmitted by amplitude and frequency modulation in the respective broadcasts. If the frame synchronizing signals are transmitted by amplitude and frequency modulation in the respective broadcasts, connections 105, 106 introduce appropriate circuits to effect the separation of the synchronizing pulses and possibly tie control of the line oscillator. Th e frequencies of the local oscillators may be such that the intermediate frequency lies between the frequencies of the carriers of the sound broadcasts corresponding to the two transmissions. The switching means 101 is arranged to insert a highly-damped tuning circuit at an appropriate frequency to affect the middle portion of the response of the intermediate frequency amplifier. In the case of the lowdefinition transmission, a slightly damped circuit having a resonance close to the highest frequency, e.g. 49 Mcs. of the corresponding transmission band so that in effect the sum of the amplitudes received by the side bands is kept constant. In a further modification, Fig. 8 (not shown), the transmission curve of the intermediate-frequency amplifier is adjusted so that the intermediate frequency corresponding to the other (high definition) broadcast carrier is located at the middle of the flank or edge opposite to that related to the highest frequency of the low-definition transmission band.